1. Field of the Invention
The present invention relates to an optical equipment with a semitransparent mirror and more particularly an optical equipment capable of eliminating double images produced by the reflected light rays from the front and rear surfaces of a semitransparent mirror disposed at an angle in the converging light flux or diverging light flux.
2. Related Background Art
When a semitransparent mirror having a pair of parallel surfaces is disposed at an angle in converging or diverging light flux, in addition to light flux reflected from one (for instance the front surface) of parallel surfaces coated with a semitransparent film, the light flux reflected from the other, transparent, surface (for instance a rear surface) not coated with a semitransparent film produces a ghost image at a position slightly displaced from the position of a real image produced by the light flux reflected from the semitransparent surface. Because of these double images, the performance of an optical equipment is degraded as is well known to those skilled in the art.
For instance, in the case of a conventional optical equipment of the type shown in FIG. 6, the light flux emitted from an object point 1 is collected and converted into a converging flux. A semitransparent mirror 3 having a pair of parallel surfaces is disposed at an angle relative to the optical axis in the converging light flux. The converging light flux is split by the semiconductor mirror 3 into reflected and transmitted light fluxes. Four light position sensors, such as a photocell which is divided into four independent areas, are mounted on a first image plane 4 upon which is focused an image by the reflected light flux. The spot-likes focused light incident positions are optoelectrically detected. Furthermore, one or more focused spots focused on a second image plate 5 is observed through a screen disposed at the second image plate 5. In the case of the optical equipment of the type described above, main light flux 10 of the normal reflected light flux reflected from the semitransparent surface (front surface) 3a is focused at one point on the image plane 4. On the other hand, the main light ray 11 (indicated by the broken line) of the light flux which is transmitted through the semi-transparent surface 3a, reflected from the transparent surface (rear surface) 3b and then transmitted through the semitransparent surface 3a is focused on the image plane at a position displaced from the position at which is focused the main light flux 10. As a result, double images are focused over the surface of the light receiving means such as a photocell so that the distribution of light quantity incident on the light receiving means varies. Thus, an error occurs in the case of the detection of a position.
Japanese Laid-Open Patent Application No. 61-271402 discloses an optical equipment in which the front surface of a dichroic mirror which reflects illumination light flux for observation and the rear surface thereof which transmits laser light flux for detection of a position are so designed and constructed that the front and rear surface are not parallel, so that interference of the laser light flux caused by reflection from the inner surface of the dichroic mirror is prevented and consequently measurement with a high degree of accuracy may be ensured.
However, in order to displace the light rays which adversely affect the correct observation out of the field of observation, when the front and rear surfaces of a flat plate are not parallel, the shape of the light path splitting portion must be made in the form of a prism having a large wedge angle. As a result, adverse bending of the optical axis of the light path is increased and the optical performance of the optical equipment is considerably degraded. Although it is possible to reduce the thickness of a semitransparent mirror to such an extent that in practice the deviation between the double image can be neglected, but there arises the problems that when the thickness is very small, the strength of the semitransparent mirror is decreased so much that fabrication becomes difficult, and the surface smoothness is also degraded.